Not all pain and sickness is from a demon, but, often when someone is dealing with pain and sickness and it's not going away, there usually is some type of involvement from the kingdom of darkness. There may not be an unlimited amount of unclean spirits, but that doesn't matter because they multitask. We see from scripture that one spirit successfully lies to 400 disobedient people at one time. Getting people to believe a lie is the enemy's greatest attack strategy.
And the Lord said, "Who will entice Ahab into attacking Ramoth Gilead and going to his death there?" One suggested this, and another that. Finally, a spirit came forward, stood before the Lord and said, "I will entice him." “By what means?" the Lord asked. "I will go out and be a deceiving spirit in the mouths of all his prophets" he said. "You will succeed in enticing him," said the Lord. "Go and do it." - 1Kings 22:20-22